1. Field of the Invention
The present invention relates to pressure sensors such as condenser microphones (or silicon capacitor microphones), which are manufactured by way of semiconductor device manufacturing processes. The present invention also relates to manufacturing methods of pressure sensors.
This application claims priority on Japanese Patent Application No. 2006-189021, Japanese Patent Application No. 2006-196578, and Japanese Patent Application No. 2006-211889, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, various types of pressure sensors such as pressure sensors of silicon capacitor types and condenser microphones, which can be manufactured by way of semiconductor device manufacturing processes, have been developed. A typical example of a pressure sensor of a silicon capacitor type is constituted of a diaphragm that vibrates due to pressure variations, a plate that is positioned opposite to the diaphragm via a dielectric such as air, and an air chamber (or a cavity). Electrostatic capacitance formed between the diaphragm and the plate varies due to vibration of the diaphragm. The pressure sensor converts variations of electrostatic capacitance into electric signals. The air chamber releases variations of internal pressure disturbing vibration of the diaphragm. Therefore, it is possible to improve output characteristics of the pressure sensor by increasing the volume of the air chamber.
Japanese Patent Application Publication No. 2004-537182 teaches a miniature silicon condenser microphone in which a cavity is formed between a recess of a substrate and a diaphragm covering the recess, wherein the internal wall of the recess is formed perpendicular to the diaphragm; hence, the opening of the recess cannot be increased to be larger than a thin film forming the diaphragm, and it is very difficult to form the cavity having a relatively large volume. Japanese Unexamined Patent Application Publication No. 2004-356707 teaches a condenser microphone in which a cavity is formed by means of a diaphragm and an internal wall of a through-hole formed in a substrate, wherein the through-hole is formed in a tapered shape whose diameter is increased in a direction opposite to a plate, so that the volume of the cavity can be increased to be larger than the volume of the cavity of the aforementioned miniature silicon condenser microphone. The tapered shape of the through-hole is formed using the lattice plane of silicon; hence, the tapered angle thereof is constant. This limits the volume of the cavity depending upon the size of a thin film forming the diaphragm; hence, it is very difficult to increase the volume of the cavity without increasing the size of the pressure sensor.
In the condenser microphone taught in Japanese Patent Application Publication No. 2004-537182 in which the cavity has a constant volume, high-frequency characteristics are degraded when the volume of the cavity is increased in order to improve low-frequency characteristics, while low-frequency characteristics are degraded when the volume of the cavity is decreased in order to improve high-frequency characteristics.
In the condenser microphone taught in Japanese Unexamined Patent Application Publication No. 2004-356707, a through-hole is formed in conformity with the two-dimensional shape of the diaphragm. This is because the through-hole serves as an introduction path for introducing an etching solution during the wet etching process, in which the prescribed portion of a sacrifice film between the thin film forming the diaphragm and the substrate in proximity to the through-hole is removed so as to form the diaphragm above the through-hole of the substrate. In the wet etching process, a part of the sacrifice film formed on the substrate is selectively removed from the substrate by way of wet etching, thus forming prescribed parts of the condenser microphone.
Due to the shape of an opening formed in the backside of the substrate opposite to the diaphragm, bubbles may occur to entirely cover the opening of the backside of the substrate during the wet etching process, thus preventing the etching solution from entering into the through-hole. Since the opening of the backside of the substrate of the aforementioned condenser microphone has a circular shape in conformity with the two-dimensional shape of the diaphragm, bubbles may easily remain in the opening due to surface tension exerted uniformly on bubbles having semispherical shapes. This makes it necessary to artificially break remaining bubbles in the aforementioned condenser microphone, which thus suffers from the complexity of the manufacturing process.